Although a heartbeat lasts just one second, it has three stages. The rate at which the heart beats is controlled by a pacemaker located in the wall of the right atrium, which sends electrical signals to all parts of the heart.
Blood flows into the atria
The heart muscle is relaxed and blood enters the upper left and right heart chambers (atria).
From atria to ventricles
The two atria contract and squeeze blood into the chambers below them (ventricles).
Blood leaves the heart
Lastly, the ventricles contract and force blood either to the lungs or around the body.
Picture Credit : Google
